The present invention relates to an on-vehicle head up display device, and more particularly to an on-vehicle head up display device utilizing a windshield glass of an automobile and indicating various information with use of a virtual image optical system.
There has been proposed a device for indicating a vehicle speed, time, engine rotational speed and other information by utilizing reflection on an inner surface of the windshield glass of the automobile and forming a virtual image at a front position of the windshield glass. Such a device is called an on-vehicle head up display device (which will be hereinafter referred to as HUD).
FIGS. 1 and 2 show a fundamental construction of the HUD in the prior art. As shown in FIG. 1, the HUD is constituted of a display unit 11 installed at a center position of a dash board, for example, a (dioptric) optical system 12 such as a lens, and a catoptric system using a windshield glass 13. The display unit 11 employs LED, LCD and the like. A small image of the display unit 11 is enlarged by the optical system 12 and the virtual image optical system formed by the windshield glass 13, and is formed as a virtual image at a front position of the windshield glass 13.
Accordingly, as shown in FIG. 2, the virtual image (indication of speed, in this case) indicated by the display unit 11 of the HUD is perceived by a driver as if the display image were present at a fixed distance in front of an automobile 14.
An image formation technique such that display information by LED or LCD and image information such as visible framework are formed in a fixed optical field is traditionally used in a camera finder, for example. A technique in the HUD is basically similar to such a display technique as mentioned above. However, the technique in the HUD includes some problems to be solved in such a point that the windshield glass is used as the catoptric system.
One of the problems is caused by the use of the windshield glass as a half mirror having a relatively high transmittance. As shown in FIG. 3, light beam directed from a point on the display unit 11 through the optical system 12 to the windshield glass 13 is reflected on two interfaces between air and glass on the outside and inside of the automobile. Further, as the optical system of the HUD is primarily provided on the dash board, it is necessary to set large an incident and reflective angle to the windshield glass. As a result, reflective images as reflected on the two reflective surfaces are slipped to generate perception of a double image.
Namely, referring to FIG. 3, the light generated from a point on the display unit 11 is allowed to enter the windshield glass in the form of beam from an object point 17 on a virtual image formed by the optical system 12. Therefore, the beam is reflected on the two reflection interfaces of the windshield glass 13, and is allowed to enter driver's eyes 16 along two paths as shown by solid lines. This is caused by refraction at the interface between glass and air on the outside of the windshield glass. Owing to such a slip of the light path, the virtual image of the object point 17 is perceived double as shown by numerals 17", and accordingly, the driver cannot visibly perceive a clear display image.
To solve this problem, it is considered that the windshield glass is treated by coating to form a mirror or half mirror having a high reflectance on an inner surface of the windshield glass. However, it is undesirable in traffic safety to provide such a mirror portion for the windshield glass of the automobile in the vicinity of the dash board. Specifically, a lower area of the windshield glass plays an important role for the driver since he watches a fender part of the vehicle body so as to grasp a condition of the vehicle body. Therefore, if the visibility of the lower area of the windshield glass is hindered, the drivability is very reduced. Furthermore, poor visibility of the windshield glass causes hindrance in quick perception of any obstacle existing in front of the automobile, resulting in traffic danger.
In addition to the aforementioned problem of double image due to double reflection, there exists another problem of binocular parallax generated primarily in dependence upon a direction of reflection.
As shown in FIG. 4, a HUD unit 1 including the display unit and the optical system is located at a central position of the dash board, and a virtual image is reflected to the driver sitting on a driver's seat of the automobile. In this case, as shown in FIG. 5, reflective points 19 on the windshield glass of the same virtual image reaching a right eye 16R and a left eye 16L of the driver are present at different positions because of binocular parallax.
FIG. 6 shows a plan view of arrangement of the driver's eyes 16R and 16L, the HUD unit (shown as an object point 1') and the reflective points on the windshield glass.
Provided that the windshield glass is a plane glass for ease of explanation, there will be now described a problem under the following conditions.
Referring to FIG. 6, an average distance L1 between the right and left eyes is about 70 mm, and a distance S1' between a center position between both the eyes 16L and 16R and a middle point between the reflective points on the windshield glass is about 850 mm. Further, as shown in FIG. 7, the virtual image is directed from the HUD unit 1 on a lower side of the eyes, and a vertical difference H between a middle point 19' between the reflective points and the eyes is about 30 mm. A distance L2 between a longitudinal line passing through the middle point 19' and the central position of both the eyes is about 210 mm. The windshield glass is inclined by 28 degrees at the middle point 19' with respect to a horizontal plane.
In other words, light beam from the HUD unit 1 located at a lower position on a left-hand side (in case of a right steering wheel) is allowed to enter the windshield glass, and the reflective light advances toward the eyes present at an upper position on a right-hand side of the reflective points.
Under the aforementioned conditions, as shown in FIG. 8, a left reflective point 19L is slipped to a left lower position from a right reflective point 19R with respect to a plane formed by a y-axis extending in a vertical direction and a z-axis extending along a transverse direction of the automobile. In this case, when the reflective points 19L and 19R are projected onto the y-z plane, a vertical difference between the reflective points 19L and 19R is 7.5 mm, while a transverse difference therebetween is 66.5 mm.
As calculated from the conditions set forth above in conjunction with FIG. 6, that is, the distance S1' (850 mm), the vertical difference H (30 mm) and the distance L2 (210 mm), a straight distance S1 between the central position of both the eyes and the reflective points is 876 mm. In this case, binocular parallax .theta. (=tan.sup.-1 (7.5/876)) in the vertical direction owing to the vertical difference of 7.5 mm between the reflective points.
Consequently, the reflective image on the windshield glass is perceived double because of the aforementioned binocular parallax, which is naturally dependent upon a transverse dimension of the image. Basically, it is hard to adjust the human eyes in the vertical direction, and accordingly, if the identical image is received by the eyes for a long time with such binocular parallax, there will occur eye strain to create a serious problem from the viewpoint of prevention of danger. Further, although curvature of the windshield glass in the vertical and transverse directions is not taken into consideration as to the binocular parallax in the foregoing description, it will be appreciated that the slip of the virtual image is increased in the case that the curvature is considered.
As is described above, the HUD utilizing the windshield glass as a reflecting means has the problems of double reflection and binocular parallax. Unless the problems are solved, a clear display image cannot be visible perceived by the driver without eye strain.